1. Field of the Invention
The present invention relates to diversity transmitters, and in particular to diversity transmitters for use in connection with mobile communication systems such as UMTS and the like. Also, the present invention relates to a corresponding diversity transmission method.
2. Description of the Prior Art
In connection with diversity transmitters, different concepts are being discussed. In general, so called open-loop concepts and closed-loop concepts can be distinguished, as it is for example outlined in the document “A Randomization Technique for Non-Orthogonal Space-Time Block Codes” by the present inventor and his co-author, presented on IEEE Vehicular Technology Conference, May 2001, Rhodes, Greece.
A number of different such open-loop concepts have been proposed in 3rd generation partnership project 3 GPP (and/or 3 GPP2). For example, in the above mentioned document “A Randomization Technique for Non-Orthogonal Space-Time Block Codes” Applicants have presented the so called ABBA concept in 3 GPP2. Motorola has proposed a combination of STTD+OTD (Space-Time Transmit Diversity+Orthogonal Transmit Diversity), and recently in the TSG-RAN Working Group 1 meeting #15 in Berlin, Germany, Aug. 22–Aug. 25, 2000, Samsung proposed a 2×STTD concept in the submitted document “New CPICH Transmission Scheme for 4-antenna transmit diversity”.
In the document “A Space-Time Coding Concept for a Multi-Element Transmitter”, by the present inventor and his co-authors, presented in the Canadian Workshop on Information Theory, June 2001, Vancouver, Canada, Applicants proposed a so-called Trombi-concept which is the subject of U.S. patent application filed on Mar. 28, 2001. The Trombi concept (explained later in some greater detail) is considered by the inventor to show currently the best performance. However, up to now the Trombi-concept was mainly implemented in connection with phase-hopping or phase sweeping arrangements. Phase-hopping and phase sweeping can be used also in the context of the present invention, but with the Trombi-method the transmission methods involving very high data rates in WCDMA downlink can be further enhanced.
Further transmit diversity concepts have been considered in the OFDM literature (orthogonal frequency division multiplexing). For example, such concepts are discussed in the document “Spatial Transmit diversity techniques for broadband OFDM systems” by S. Kaiser, published in IEEE, 2000, page 1824–1828, (0-7803-6451-1/00).
This proposed concept by Kaiser however requires interleaving over multiple frequencies for full benefit. A similar approach is discussed in U.S. Pat. No. 6,157,612. Moreover, according to the teaching of Kaiser, a symbol to be transmitted is distributed across several carriers, so that for combining the received multipath components, guard intervals are required in order to be able to correctly combine the transmitted (distributed) symbol parts at the receiving side
Referring back to the above mentioned so-called Trombi concept the following was proposed. A time-varying/hopping phase (for example pseudo-random) is added to the dedicated channel of a given user at the output of STTD encoder (Space-Time Transmit Diversity) (or an encoder based on some other orthogonal or non-orthogonal concept, see for example the previous “Randomization technique . . . ” paper).
In one solution with 4 antennas, antennas 2 and 4 are multiplied by a complex coefficient (constant for two space-time coded [successive] symbols) to result in the following received signal. The received signal r, the symbols S, the transmission channel transfer functions h and complex coefficients w are generally given in matrix notationrt1=S1(h1+w1(t)h2)−S2*(h3+w2(t)h4)rt2=S2(h1+w1(t)h2)+S1*(h3+w2(t)h4)  (1)
In a preferred arrangement, it is configured such that w1 (t)=−w2 (t), with constant amplitude=1. Phase changes according to a suitable pseudo-random sequence. For example, it can hop with phases 0, 180, 90, −90, (or with any other sequence) known [a priori] to the terminal (receiver). 8-PSK hopping appears to be sufficient to get achievable gains.
Then, the terminal estimates the channels h1, . . . , h4, for example using common channel pilots (or dedicated pilots) which do not need to apply phase dynamics (for example common channel measurements can be done as proposed by Samsung in the cited document). Alternatively, the terminal can measure the effective channels h1+w*h2 and h3−w*h4 only.
By knowing the channels and the pseudo-random weights at the transmitter the intentional phase dynamics can be taken into account and then the detection reduces to conventional STTD decoding without any complexity increase.
In essence, the dynamics of the phase-hopping should be a priori fixed or at least it should be known by the UE (for example by suitable signaling from the transmitter to the receiver). In some cases it may also be advantageous if the UE controls the phase-hopping sequence.. As such a control procedure is expected to be known to those skilled in the art, these details are supposed to be not needed to be explained here.
With channel coding, providing time diversity, the concept has better performance in low Doppler channels than a two antenna STTD concept, as shown in the “Trombi paper”. Phase-hopping diversity can be used also in a way such that the channel estimates are directly taken from a phase-hopping channel. In that case the hopping sequence can have only incremental changes, as otherwise the effective channel is changing too rapidly to enable efficient channel estimation. However, in this case the receiver terminal (User Equipment UE in UMTS) does not necessarily need to know that phase-hopping is used at all.
Therefore, in the aforementioned scheme, phase-hopping can weaken channel estimation performance by the abrupt phase hops, or the hops have to be quantized to many levels, to thereby approximate a phase-sweep.
The Trombi concept is designed for sequential transmission, and the phase-hopping sequence is defined over multiple time instants, covering multiple space-time encoded blocks. In future communication systems the whole information frame may be transmitted in one or a few symbol intervals (for example if in a CDMA system essentially all downlink codes are allocated to one user at a time). In such an extreme case, only one or a few phase-hopping values can be incorporated to the transmission, and the benefits of the Trombi concept cannot be achieved.
As an example, in “Draft Baseline Text for Physical Layer Portion of the 1×EV Specification” 3 GPP2 C.P9091 ver. 0.21, Aug. 24, 2000 (3 GPP2 TSG-C working group III) the physical layer of the High Data Rate CDMA system is described. This system uses Time Division Multiplexing in downlink and each user can be allocated only one slot, and the pilots are structured so that only one channel estimate can be obtained for this one slot.